1. Field of the Invention
This invention relates to a spindle drive unit comprising a spindle having a spindle axis and an external circumferential surface, a nut embracing said spindle and being in threaded engagement with said spindle and at least one wiper assembly comprising at least one wiper member in wiping engagement with said external circumferential surface of said spindle, said wiper assembly being removably fastened to said nut in an axial end portion thereof.
2. Description of the Prior Art
A spindle drive unit like this is known from the DE 28 22 560 A1. In this known solution the wiper assembly comprises a sealing ring attached to the internal circumference of a sleeve. The sealing ring sealingly abuts on the back of an external thread of the spindle. The sealing ring is permanently riveted to the sleeve. The wiper assembly consisting of the sealing ring and the sleeve may be fixed to the nut when the nut is mounted on the spindle. For this purpose an axially projecting tubular socket is welded on the nut. The sleeve of the wiper assembly supporting the sealing ring is pushed on said tubular socket. Then the wiper assembly is manually adjusted in the axial direction with respect to the tubular socket until the sealing ring is optimally positioned on the back of the external thread of the spindle. After adjusting the wiper assembly, it is axially and circumferentially fixed to the tubular socket by means of pins and a clamp strap.
For this known solution an adaptation of the nut is necessary to be able to mount the wiper assembly. The nut has to be subjected to a welding process to attach the tubular socket to it. If a conventional nut, which can be obtained as a component of mass production, is to be used, it is often completely impossible to weld a tubular socket from the outside on the nut. Often the material of the nut or the components connected therewith do not allow welding of a tubular socket, for example due to high sensitivity to heat, great danger of deformation or poor welding properties. This especially applies to precision spindle drive units, which are used in machine tools or measuring devices. Moreover, welding is a comparatively extensive and difficult process as the tubular socket has to be positioned exactly in the radial direction in order to guarantee uniform abutment relations of the sealing ring at the back of the spindle thread across the entire circumference. Even in conventional nuts often no suitable place for welding the tubular socket is found for reasons of design or space. Finally, the tubular socket is no longer removable from the nut after welding so that the nut is assigned to a certain purpose of application and can no longer be used for other applications in which a wiper assembly axially attached from the outside is under certain circumstances not necessary anymore. In this case the welded tubular socket would be disturbing and require unnecessary space.
From U.S. Pat. No. 3,643,521 there is known a further spindle drive unit having a spindle and a nut being in threaded engagement with the spindle. In a mounted state of the nut a sealing ring can be attached to the nut from axially outside. The sealing ring engages with a threaded groove in the external circumferential surface of the spindle and is screwed on the spindle from one end of the spindle. A plurality of axially oriented peg receiving holes are provided in an axial end face of the nut distributed along the circumference. Similar peg receiving holes are also provided in the sealing ring. If the sealing ring, when mounted on, is turned into its desired axial position relative to the nut, the peg receiving holes in the sealing ring and the nut are aligned and then equipped with pegs which keep the sealing ring to the nut without being able to be turned. By means of a pretensioning member supported on the nut the sealing ring is axially biased away from the nut against the turn of the spindle thread.
Even in the solution according to DT 20 01 558 a conventional and commercially available nut cannot readily be used to attach the sealing ring. It is rather that the nut has to be purposefully machined to create the preconditons for mounting the sealing ring. For the peg receiving holes have to be drilled which causes an additional expenditure of work. Moreover the mounting of the sealing ring is extraordinarily laborious since the sealing ring has to be held against the action of the pretensioning member relative to the nut to be able to insert the individual pegs into the peg receiving holes which have been brought in alignment.